This invention relates to apparatus for converting energy present in surface waves on bodies of water to useful energy, and particularly to means for protecting such apparatus from storm induced surface turbulence by either raising the apparatus above or sinking it below the water surface.
Wave energy converters, referred to hereinafter as WECs, are known and described, for example, in co-pending application Ser. No. 10/762,800, filed Jan. 22, 2004, the subject matter of which is incorporated herein by reference. In the co-pending application, there are described two floats, one having an annular or tire-like configuration and floating in generally horizontal orientation. The other float is elongated (referred to hereinafter as a spar) and floats in vertical orientation inside the central opening of the annular float. Both floats bob up-and-down in response to passing surface waves, but generally in an out-of-phase relationship. When the annular float, for example, is rising, the spar generally tends to be sinking. The relative movements between the two floats are used for driving an energy converter, such as a linear electrical generator, for generating useful energy.
A problem associated with the use of a WEC disposed near or on the surface of a body of water is the danger that excessively large waves can cause damage to the WEC. A known practice for protecting a WEC in storm conditions is to sink it to a depth below the surface zone of turbulence. While such deliberate sinking of the WEC can be done by flooding a ballast tank, as in a submarine, this requires elaborate and expensive apparatus including a source of pressurized air for blowing the flooded tanks.
Another technique for sinking a WEC comprises winding an anchoring cable of the WEC around a motor driven drum on the floor of the water body and forcibly dragging the WEC to a safe depth. A problem here, however, is that for highest energy generating efficiency, the WEC preferably has substantial reserve buoyancy (i.e., is subject to a substantial buoyant force when the instantaneous water surface is elevated relative to the calm condition waterline of the WEC). But the greater the reserve buoyancy of the WEC, the greater is the force required not only to sink the WEC but for controlling its rate of ascent when the WEC is resurfaced. The greater the sinking and elevating forces, the larger must be the overall system including an anchor of sufficient strength for withstanding the applied forces, and the more complex must be the mechanisms to hold the WEC in and release the WEC from a submerged state.
An alternative practice for protecting a WEC, usable in situations where the WEC is suspended from a support structure, for example, an ocean platform, is to pull the WEC upwardly out of the zone of influence of the waves. There is a problem in this approach which is analogous to the problem of submerging the WEC: for the WEC to be efficient, it has to displace a substantial weight of water, because this displaced weight is approximately equal to the maximum force experienced by the WEC when the instantaneous water surface drops below the calm condition waterline. The substantial weight required for efficient wave energy conversion however, poses onerous requirements on the mechanisms required to pull the WEC upwardly out of the water and to eventually release the WEC in a controlled manner.
The present invention is directed to means for reducing the amount of force required for moving a WEC from its normal surface floating position to a position of safety.